The present invention is directed to a timing acquisition technique for synchronizing a receiving modem with a transmitting modem and, in particular, to speed up the timing acquisition so that it can be completed during a short sequence of a training procedure.
A modem, i.e. a modulator and demodulator, is designed to both transmit and receive data over a transmission line, such as a telephone line, during a period of time called a "connection" which begins when an answering modem goes off-hook in response to a call from a calling modem, and which ends when an on-hook condition is restored. A connection typically includes one or several communication sessions, depending on whether the modem is full-duplex, half-duplex, or a half-duplex emulating a full duplex. Each communication session is a single transmission of a training sequence followed by information data (as explained below) between the transmitter and the-receiver modems.
In a full-duplex modem, the modems at both ends of the line transmit and receive simultaneously. A communication session starts with both modems training each other, followed by information data (as explained below). A true half-duplex modem, such as is used for fax transmissions, is either a transmitter or a receiver throughout the connection period. A special application of a half-duplex modem is to emulate a full-duplex modem. This is done by efficient (fast) switching of roles of transmitter and receiver between the two sides, and it allows a system having a half-duplex modem to behave as though it has a full-duplex modem at its disposal. Both modems can swap roles of transmitter and receiver as many times as needed during a connection, with the respective roles being fixed during a communication session.
The present invention applies in particular to a half-duplex modem which emulates full-duplex operation. Data which is transferred, or communicated, from one modem to another during a session is referred to herein as "information data". However, signals other than the information data must also be transferred in order for the modems at the transmitting and receiver ends to cooperate properly. In particular, various parameters of the receiver modem must be set at the initiation of a communication session so that the data is transferred accurately. A training procedure is used for conditioning a receiver modem, and the sequence of bits associated therewith is referred to herein as a "training sequence".
Standards have been established by a number of organizations which define various forms of communication via modem so that dissimilar equipment made by different manufacturers will interact to provide the required result of transferring information data accurately and reliably.. One such organization is the International Telecommunication Union (ITU). Its V.27ter and V.29 standards apply to 4800/2400 bps (bit per second) and 9600/7200 bps modems, respectively. These standards define QAM (quadrature amplitude modulation) modems which are applicable to half-duplex operation, for example. According to these standards, a training procedure which involves two different types of training sequences is used to properly condition the receiver modem. Of the two sequences, the first type is a longer one which is used only for the first session when a receiver modem receives a transmission during the connection. The second type of sequence is shorter, and it is used for the subsequent and all ensuing sessions of that same connection to make the turn-around (i.e. as the transmitter and receiver modems swap roles, and data starts flowing in the opposite direction) time shorter.
FIG. 1 is a block diagram of known steps performed to initiate a connection by a prior art receiver modem which is of the half-duplex type that emulates full duplex operation. The modem goes off-hook in step 1 in answering a ring signal. The training procedure begins with step 3 which receives and processes the above-mentioned long training sequence to make, inter alia, a timing determination, and step 5 follows to suitably adjust the timing in accordance with the results obtained by step 3. Step 6 then receives the transferred information data in a conventional manner. In fact, however, steps 5 and 6 may be combined because the long training sequence and the information data can both be received as part of the same data block which includes the following: ##STR1##
The actual digital data block arrangement used depends on which one of many different protocols (eg. BISYNC, SDLC, HDLC) is adopted.
When the communication session ends, step 7 switches the role of the receiver modem to a transmitter, as needed, and then transmits per step 8. The newly designated receiver will then undergo steps 3, 5, 6 and 7, although these steps are not shown because FIG. 1 applies only to control of one modem. When that session ends, step 9 returns the modem to its receiving role (i.e. modem turn-around).
If it is determined by step 10 that more information data remains to be transferred from one side of the transmission line to the other during the current communication session, then step 11 receives and processes the short training sequence, and step 13 adjusts the receiver modem parameters accordingly. The flow returns to step 6 for receiving the additional information data. Steps 6 and 11 can also be combined for the same reason given above regarding the combination of steps 5 and 6. If, however, step 10 determines that all the information data has been transferred, then the connection is terminated as the modem hangs up per step 14.
The two types of training sequences each consist of the following five segments.
TABLE 1 ______________________________________ 1 2 3 4 5 ______________________________________ V.27ter: Short 85-200 ms 20-25 ms 14 SI 58 SI 8 SI Long 85-200 ms 20-25 ms 50 SI 1074 SI 8 SI V.29: Short 185-200 ms 20 ms 100 SI 62 SI 18 SI Long 185-200 ms 20 ms 128 SI 384 SI 48 SI ______________________________________
where SI is a symbol interval. A "symbol" is the basic unit of information transmitted by the modem (sometimes also referred to by the term "baud"). V.29 modems send 2400 symbols/second with each symbol having 4 bits or 3 bits, thus yielding 9600 or 7200 bps, respectively. V.27ter modems send either 1600 symbols/second with each symbol having 3 bits, thus yielding 4800 bps, or 1200 symbols/second with each symbol having 2 bits, thus yielding 2400 bps. The training sequence is also defined in terms of symbols.
The task of each segment of the training sequences is well defined in the ITU standards and, therefore, need not be described here in detail. Suffice it to say for the purpose of describing the present invention that segment 3 (as defined in Temporary Document 23-E of the ITU, or the CCITT as it was then known, titled "Liaison Statement to Study Group XVII on Short Train Facility Within V.29") performs, inter alia, timing acquisition in order to synchronize the two modems.
The analog signal received over the transmission line is typically demodulated and detected by digital techniques which involve analog to digital (A/D) conversion as a first step. The sampling method of the A/D conversion can be used to assist the receiver modem to correctly detect the received symbol. In particular, the symbol boundaries are determined accurately by the receiver modem, and then the decision as to which symbol has been received is made at the peak of the symbol power where the accuracy of the decision is maximal and best modem performance can be achieved. The receiver modem uses the feature that the sampling time, or instance, can be set in the A/D converter in order to move the samples, as needed, such that the modem can perform detection at the peak of symbol power.
In the long training sequence, a coarse timing algorithm is performed to determine the deviation from the ideal point at which the received signal should be sampled. The short training sequence also applies a coarse timing algorithm and, in addition, a fine timing algorithm is used to make a timing adjustment. It is feasible to make its segment 3 shorter because the short training sequence benefits from reliance on information gathered in previous sessions, such as the typical gain to use, equalizer coefficients, etc. A considerable time saving can thus be achieved when timing adjustments for a given communication session are made by using only the short training sequence, as is readily apparent from Table 1. Additional timing acquisition steps to achieve synchronization may be performed during transfer of the information data, but this is not a part of the present invention.
During segment 3 of the long training sequence there is sufficient time to make an initial coarse estimate of the correct symbol timing, and then to converge the modem toward perfect synchronization. However, time is scarce during the corresponding segment 3 of a short training sequence. If the modem fails to converge within the time available in a short training sequence, it will consequently make errors during the ensuing reception of information data. The higher the modem speed, the shorter the symbol is and, thus, the receiver modem becomes more vulnerable to synchronization errors. Therefore, fast synchronization is especially important for the higher speeds (e.g. 9600 bps).